To study the effect of Type C RNA viruses on hematopoiesis with particular emphasis being placed on the spontaneous disease in AKR mice. Major emphasis will be placed on the preleukemic phase of the disease, i.e., hematopoietic changes occurring during the first 3 months of life. To contrast animal data in Rauscher virus leukemia and AKR leukemia with similar studies being conducted in human beings with illnesses that mimic the virus diseases in mice, i.e., human acute leukemia; the myeloproliferative diseases, idiopathic thrombocytopenic purpura, and immunohemolytic anemia. To study the effect of specific drugs on the preleukemic phase of the disease in experimental animals to ascertain whether therapy at this point might abort the ultimate development of the leukemic state.